1. Field of the Invention
The present invention relates to implantable heart stimulators, such as pacemakers, defibrillators, cardioverters, implantable cardioverter-defibrillators (“ICDs”), and similar cardiac stimulation devices that also are capable of monitoring and detecting electrical activities and events within the heart.
2. Description of the Prior Art
Implantable pacemakers generate electrical stimulation pulses and deliver such stimulation pulses to atrial and/or ventricular muscle tissue of a patient's heart at a prescribed rate and/or rhythm when, through disease or other cause, the heart is not able to maintain the prescribed heart rate or rhythm on its own. When the delivered electrical stimuli are of sufficient energy, they cause the cardiac muscle tissue to depolarize, and therefore contract, thereby forcing the heart rate or rhythm to track the delivery of the electrical stimuli. When the delivered electrical stimuli are of insufficient energy, depolarization does not occur, and the heart rate or rhythm is not controlled by the pacemaker. Hence, for the pacemaker to perform its intended function, it is important that the delivered electrical stimuli is of sufficient energy to depolarize the cardiac tissue.
The depolarization and ensuing contraction of the heart in response to a delivered cardiac stimulation pulse is generally referred to in the art as “capture”. Consequently, the term “non-capture” denotes the condition when a delivered stimulation pulse does not result in depolarization and contraction. When detecting capture, a sensing circuitry checks for the depolarization of a cardiac chamber following and in response to a delivered stimulation pulse. Such a depolarization as a result of a delivered stimulation pulse is also referred to as an “evoked response” (ER) of that chamber. Furthermore, the evoked response is detected during a selected time period following the delivery of a stimulation pulse. Such a time period is generally referred to as an “evoked response window”.
The energy of the electrical stimuli generated by an implanted pacemaker is derived from the energy stored in the pacemaker power source or battery. The pacemaker battery has a limited amount of energy stored therein, and the generation of electrical stimuli represents by far the greatest drain of such energy.
The amount of energy needed to effectuate capture is known as the capture “threshold”, and electrical stimuli of energy less than the capture threshold do not bring about capture, while electrical stimuli of energy greater than the capture threshold do bring about capture.
A capture threshold search normally begins at a desired starting point (either a high energy level or the level at which capture is currently occurring) and the energy level is decreased until capture is lost. The value at which capture is lost is known as the capture threshold. Thereafter, in order to secure capture, a safety margin is added to the capture threshold to arrive at the energy content of the stimulation pulse. One of the key issues is to choose the safety margin such that it guarantees capture and at the same time provides adequate energy savings and does not cause pectoral stimulation and/or sensation.
In order to preserve this limited energy and to prolong the life of the battery, it is known in the art to adjust the energy content of the delivered electrical stimuli so that it is just sufficient to cause capture, with an appropriate safety margin. One such method is described in U.S. Pat. No. 6,714,819, wherein a calculation of the safety margin is made based on a measured variation of the threshold value. However, when such calculation are used the adjustment of the safety margin is not immediate, but dependent on previous threshold values. Hence, there will be a delay in the adjustment of the safety margin, if a rapid change suddenly occurs after a period of substantially constant threshold values.
Capture detection may occur on a beat-by-beat basis or on a sampled basis. In autocapture systems where a beat-by-beat capture verification is provided by the pacemaker, a fixed safety margin has generally been proven to be adequate. If a threshold increase occurs, the pacemaker responds by increasing the output amplitude and re-finding the capture threshold, to which the safety margin is added. This method reduces the energy consumption in the stimulation device by adding the lowest possible safety margin that the pacemaker allows.
For example, in a cardiac stimulation device arranged for biventricular stimulation, the stimulation threshold and the evoked response, or depolarization of the muscle tissue, are measured both in the first and the second stimulated ventricle, which normally is the left and the right ventricle, respectively. In many cases, there is an intraventricular delay between the stimulation pulses delivered to the first and the second ventricle. When this delay is short, e.g. less than approximately 60 ms, the stimulation pulse of the secondary ventricle can interfere with the capture detection in the primary ventricle. This interference makes the evoked response detection in the primary ventricle difficult.
Therefore, capture is not always verified on a beat-by-beat basis in the primary ventricle, but rather after certain programmable time intervals, e.g. 15 min. or 1000 heartbeats. At these instants, the evoked response (ER) window for the first ventricle is normally made clear from disturbing stimulation pulses in other places of the heart by a temporarily changed timing pattern.
To ensure capture between these threshold verifications, a sufficient, fixed safety margin is introduced for the first ventricle. This safety margin is normally higher than the safety margin in the second ventricle to account for the fact that the capture verification is not performed on a beat-by-beat basis in the first ventricle.
However, capture verification requires additional processing time and corresponding consumption of battery energy.